The adaptation of man and other animals to a planet with a 24 hour period of rotation has resulted in the evolution of a circadian timekeeping system that organizes the physiology and behavior of organisms. Individuals exposed to altered gravitational or temporal environments often exhibit physiological or behavioral deficits. In spaceflight nonhuman primates exhibit a depression of body temperature, free-running circadian body temperature rhythms, and substantial changes in sleep-wake behavior. Metabolic and psychologic changes resulting from desynchronized circadian rhythms may significantly affect the health and well being of astronauts and cosmonauts. This project continues our investigation of the biomedical effects of alterations in gravity and lighting on metabolism and circadian rhythms of body temperature, heart rate, activity, feeding, drinking, and behavior of the rhesus monkey. These data may suggest interventions that would alleviate adverse human responses to conditions such as spaceflight, jet-lag or shift work. We recently measured the metabolic energy expenditure of three monkeys flown in space for 14 days . Measurements of energy expenditure were obtained from the oral (nasogastric intubation) administration of the nonradioactive stable isotope 2h218o (double labeled water). Analysis of the labeled water in excreted urine provide an index of metabolic rate and total body water. The results from these animals indicate a reduction of 40% energy expenditure during spaceflight as compared to levels on earth.